HEAT .TREATMENT. '303
,to mix with'the ferrite to form pearlite, and consequently leave
more ferrite free. In No. 40 is shown the structure of an ingot
containing 0.47 per cent, of carbon magnified 20 diameters. No.
41 gives the structure of an 8" bloom rolled from a 32/-x38" ingot,
and No. 42 a test from the same bloom hammered to a piece 2"
.square. .These last two are magnified 75 diameters, and it should
be noted that the areas of the ingot structure shown in the photo-
;graphs are to the areas of the finished pieces as one to fourteen.
';.Figs. 44 and 45 show the.structure of a steel,containing about
vone per cent, of carbon before and after rolling, the first being a
section'from a 16"x20" ingot., the latter a section from a piece 1" in
.diameter cooled on the hot bed, It will be seen that the grain is
'well broken up without any sign of cold work, and the bar is con-
sequently in very good condition for the hardening and tempering
^o which such hard steels are usually subjected. This bar;was
Tfcaken at random from the hot bed at Steelton. : .
: If steel is worked below the critical point, "strains are developed
which injure the metal and may even rupture it. In No. 9, Pig.
XV-B, is shown a piece of forging steel magnified 30 diameters.
, This illustrates the distortion of'cold work, and the black line in
.the middle of the print is a crack where the tension became greater
i than, the cohesion of the metal.
. SEC. XVI.Effect of work upon the structure of rails.Nos. 19 .and 20,, in- Fig. XV-D, show the micro-structure of two rails which broke in service. No data are available as to how long they had . been in use, but it is probable that it was only a short time. No. 21 is an 85-lb. T rail,'which broke under the drop test. These three .fractures, a's well as all the other photographs, are selected not as exceptional, but as representative of what will usually be found under similar conditions. Fig. 22 is made from a heavy rail section . finished at a temperature of 1000° C., and it will be noticed that its appearance is almost if'not .quite the same as that of NTos. 19, 20 and 21. In Nos. 23, 34, 34 and 35 are shown the results of some experiments performed by Mr. S. S. Martin at the works of the Maryland Steel Company at Sparrow's Point. An ingot was rolled into blooms and two adjacent blooms were rolled into rails without further heating, the first being held before rolling in order to allow it to cool so that all work should be done at as low a temperature "as possible, without, of course, reaching the lower critical point, while the second was rolled as quickly as possible through all the